In recent years, due to concerns about the environment, solar cells have been attracting a growing interest. In particular, attention has been drawn to chalcopyrite solar cells which are thin-film solar cells with high photoelectric conversion efficiency, and research and development have been actively conducted.
A chalcopyrite solar cell is produced by forming a light-absorbing layer prepared from a chalcopyrite material on a substrate. Representative elements of a chalcopyrite material include copper (Cu), indium (In), gallium (Ga), selenium (Se) and sulfur (S). Representative examples of the light-absorbing layer include Cu(In,Ga)Se2 and Cu(In,Ga)(Se,S)2, which are referred to as CIGS and CIGSS, respectively.
FIG. 1 is a schematic cross-sectional diagram of an example of a chalcopyrite solar cell.
As shown in FIG. 1, a chalcopyrite solar cell has a basic structure in which a first electrode 3, a CIGS layer 4 (light-absorbing layer), a buffer layer 5, an i-ZnO layer 6 and a second electrode 7 are laminated on a substrate 2 in this order. As the buffer layer, for example, a CdS layer, an ZnS layer and an InS layer are known.
Each of the first electrode 3 and the second electrode 7 has a terminal connected thereto, and each of the terminals is connected to a wire. In such a chalcopyrite solar cell 1, an incident light entering in the A direction is absorbed by the CIGS layer 4 to generate an electromotive force, and an electric current flows in the B direction.
The surface of the second electrode 7 is covered with an antireflection film layer 8 composed of, for example MgF2.
As a method of forming a CIGS layer 4, a sputtering method and a coating method are known. However, when a sputtering method is used, the apparatus has to be scaled up, which resulted in deterioration of the yield. For this reason, diligent studies have been made on the coating method which enables production at a relatively low cost.
Generally, in a coating method, elements such as Cu, In, Ga, Se and S are dissolved in a specific solvent to prepare a coating solution, and the coating solution is applied to a substrate by a spin coating method or a dipping method, followed by baking, thereby forming a CIGS layer.
In the preparation of a coating solution, there are known a method in which hydrazine is used as the solvent, and a method in which amine is added as a dissolution promoter instead of using hydrazine.